An X-Y touchscreen using a relatively high density array of sensing nodes can easily sense the presence of a finger. In some cases, the nodes may have a center to center pitch of 5 mm or less. Use of a passive stylus on the touchscreen presents a very small detection patch with a relatively weak central signal as compared to a finger touch. For capacitive sensors, the weak signal results from a reduced surface area of the stylus. For optical sensors, there may be less of a dark spot under the stylus tip due to light leakage around the edges of the stylus. A complete shadow may not be observable with the stylus as compared to a finger touch.
In some prior touchscreen devices, different thresholds may be selected depending on whether a finger or a stylus are most likely to be used. The thresholds may be used to determine whether the touchscreen is being initially touched, and results in further touches being detected and interpreted as touches even if they are below the initial threshold. This is referred to as the touchscreen entering a detect mode once it is initially determined that the touchscreen is being touched. The initial threshold may be set according to whether it is expected that a finger or a stylus will be used.
In one prior device, the threshold is a function of the measured capacitance at each node which changes in response to a touch by a finger or stylus. The capacitive measurement is compared to the threshold, and if exceeding the threshold, a detect state is entered. The detect state is maintained as long as the measured capacitive changes on the nodes exceed the threshold. Commonly, the threshold is slightly reduced on entering the detect state to ensure that the detection state is maintained even if the capacitive change drops slightly due to measurement uncertainty or noise for example. The original threshold is restored once the detect state is exited due to removal of the touch. This is a form of hysteresis.
If a user switches between using a finger and a stylus, the initial threshold may not be set appropriately by the user or by the product designer. If the threshold is set for a stylus, the touchscreen may be too sensitive if a finger is used and noise may also result in touches incorrectly being detected. Conversely if the threshold is set to optimal for finger use then it may not detect a stylus consistently, or at all.